This application is based on, and claims priority to, Japanese patent application number 07-304229, filed on Nov. 22, 1995, in Japan, and which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical communication system using optical phase conjugation to suppress waveform distortion caused by chromatic dispersion or group velocity dispersion (GVD) and the optical Kerr effect and, more particularly, to the use of a polarization maintaining fiber in such an optical communication system and to the transmission of a wavelength division multiplexed (WDM) light signal in such an optical communication system.
2. Description of the Related Art
Optical communication systems typically transmit optical signals through a fiber (or xe2x80x9coptical fiberxe2x80x9d) over long distances of several hundred to several thousand kilometers, and with transmission speeds as high as several Gb/sec to 10 Gb/sec or more. However, the transmission quality of the optical signals travelling through the fiber can significantly deteriorate due to GVD in the fiber. As a result, the transmission speed and/or transmission distance is usually limited by the effect of GVD.
Moreover, intensity modulation or amplitude modulation is used to produce a signal light having a waveform of optical pulses. However, the resulting waveform may be distorted for reasons other than GVD. For example, waveform distortion can be caused by GVD and the optical Kerr effect.
The following is a discussion of the relationship of GVD to the optical Kerr effect. Assume that an optical pulse propagates in a dispersive medium. When an unchirped pulse passes through a normal dispersion medium (∂2xcex2/∂xcfx892 greater than 0, where xcex2 and xcfx89 denote the propagation constant and the angular frequency of the light, respectively), the pulse is shifted toward a lower frequency side at its rising edge and is shifted toward a higher frequency side at its falling edge. By contrast, in the case of an abnormal dispersion medium (∂2xcex2/∂xcfx892 less than 0), the pulse is shifted toward a higher frequency side at its rising edge and is shifted toward a lower frequency side at its falling edge.
Further, in a normal dispersion medium, the longer the wavelength, the higher the group velocity. By contrast, in an anomalous dispersion medium, the shorter the wavelength, the higher the group velocity. Therefore, in either case, the pulse width is increased.
When the light intensity is great, the refractive index is changed by the following value due to the optical Kerr effect.
Equation (1):
xcex94n(t)=n2|E(t)|2 
In the above Equation (1), n2 is the nonlinear refractive index. For example, with a silica fiber, the nonlinear refractive index n2 is approximately 3.2xc3x9710xe2x88x9220 m2/W.
When an optical pulse is affected by the optical Kerr effect in a nonlinear medium, the spectrum is chirped as shown by the following Equation (2).
Equation (2):       Δω    ⁡          (      t      )        =            -                        ∂                      ΔΦ            ⁡                          (              t              )                                                ∂          t                      =                  -                              2            ⁢            π            ⁢                          xe2x80x83                        ⁢                          n              2                                λ                    ⁢                        ∂                                    "LeftBracketingBar"                              E                ⁡                                  (                  t                  )                                            "RightBracketingBar"                        2                                    ∂          t                    ⁢      Δ      ⁢              xe2x80x83            ⁢      z      
where xcex94z denotes the interaction length. This phenomenon is generally termed self-phase modulation (SPM).
Due to SPM, the optical pulse is shifted to a lower frequency side at its rising edge and is shifted toward a higher frequency side at its falling edge. Because of the chirping caused by such SPM, the influence of the dispersion is rendered more noticeable, thereby increasing the pulse distortion.
Therefore, when the optical pulse is affected by the optical Kerr effect in a normal dispersion medium, the pulse broadens more than in the case of dispersion alone. Moreover, in an anomalous dispersion medium, pulse compression occurs. An optical soliton is obtained by counter-balancing GVD and SPM.
In an anomalous dispersion medium, a high signal-to-noise ratio can be advantageously retained by applying pulse compression derived from SPM. However, recent research developments enable satisfactory transmission with high-level optical power by the use of an optical amplifier and a dispersion-shifted fiber (DSF) having a relatively small GVD. Therefore, it is uncertain whether pulse compression will provide improved results. In other words, large waveform distortion is generated as the pulse compression effect is rendered excessive. Particularly in the case of non-return to zero (NRZ) pulses, concentrative pulse compression occurs at rising and falling portions of the pulses, so that severe waveform changes are induced. In an extreme case, a falling portion passes a rising portion to eventually cause a phenomenon that one pulse is split into several pulses.
In view of the above, significant waveform distortion occurs from GVD and the optical Kerr effect. Such waveform distortion can be reduced by using conventional fibers having approximately zero dispersion at wavelength regions of 1.3 xcexcm and 1.55 xcexcm. However, without other countermeasures and due to GVD and the optical Kerr effect, such fibers may not adequately suppress waveform distortion.
Accordingly, it is an object of the present invention to provide an optical communication system, such as a wavelength division multiplexing (WDM) optical communication system, which suppresses waveform distortion caused by GVD and the optical Kerr effect.
It is a further object of the present invention to provide an optical communication system which uses optical phase conjugation (OPC) and a polarization maintaining fiber to reduce variations of polarization, to thereby suppress waveform distortion derived from GVD and the optical Kerr effect.
It is an additional object of the present invention to provide an optical communication system which maintains an optimal state of reception despite variation of polarization in the optical communication system.
Objects of the present invention are achieved by providing an apparatus for transmitting a light signal. The apparatus includes a first fiber, a phase conjugator, and a second fiber. The first fiber transmits the light signal therethrough, and is a polarization maintaining fiber. The light signal is a linear polarized wave. The phase conjugator receives the light signal from the first fiber and produces a corresponding phase conjugate light signal. The second fiber receives the phase conjugate light signal from the phase conjugator and transmits the phase conjugate light signal therethrough.
Objects of the present invention are also achieved by (a) setting the amount of dispersion of the first fiber to be substantially equal to the amount of dispersion of the second fiber, and (b) setting the amount of the optical Kerr effect of the first fiber to be substantially equal to the amount of the optical Kerr effect of the second fiber.
Objects of the present invention are also achieved by (a) setting the ratio of the dispersion of the first fiber to the dispersion of the second fiber to be substantially equal to the ratio of the length of the second fiber to the length of the first fiber, and (b) setting the ratio of the product of the optical frequency, the light intensity and the nonlinear refractive index of the first fiber to the product of the optical frequency, the light intensity and the nonlinear refractive index of the second fiber to be substantially equal to the ratio of the length of the second fiber to the length of the first fiber.
In addition, objects of the present invention are achieved by dividing the first fiber into a plurality of sections which each have an associated dispersion and an optical Kerr effect, and dividing the second fiber into a plurality of sections which each have an associated dispersion and an optical Kerr effect and which correspond, respectively, to the plurality of sections of the first fiber, wherein (a) the amount of dispersion in each section of the plurality of sections of the first fiber is set to be substantially equal to the amount of dispersion of the corresponding section of the plurality of sections of the second fiber, and (b) the amount of optical Kerr effect in each section of the plurality of sections of the first fiber is set to be substantially equal to the amount of optical Kerr effect of the corresponding section of the plurality of sections of the second fiber.
Further, objects of the present invention are achieved by dividing the first fiber into a plurality of sections which each have an associated dispersion, length, light intensity, and nonlinear refractive index, and dividing the second fiber into a plurality of sections which each have an associated dispersion, length, light intensity, and nonlinear refractive index, and which correspond, respectively, to the plurality of sections of the first fiber. The ratio of the dispersion in each section of the plurality of sections of the first fiber to the dispersion of the corresponding section of the plurality of sections of the second fiber is set to be substantially equal to the ratio of the length of the respective section of the plurality of sections of the second fiber to the length of the respective section of the plurality of sections of the first fiber. In addition, the ratio of the product of the optical frequency, the light intensity and the nonlinear refractive index of each section of the plurality of sections of the first fiber to the product of the optical frequency, the light intensity and the nonlinear refractive index of the corresponding section of the plurality of sections of the second fiber is set to be substantially equal to the ratio of the length of the respective section of the plurality of sections of the second fiber to the length of the respective section of the plurality of sections of the first fiber.
Objects of the present invention are also achieved by providing a monitoring unit which monitors a parameter of the phase conjugate light signal transmitted in the second fiber and indicating the reproduction quality of the transmission data, and controls at least one of the wavelength and power of pump light of a pump source of the phase conjugator to optimize the reproduction quality of the transmission data.
Moreover, objects of the present invention are achieved by providing an apparatus which uses phase conjugation to transmit a wavelength division multiplexed light signal. More specifically, a first fiber transmits the wavelength division multiplexed-light signal therethrough. A demultiplexer receives the wavelength division multiplexed light signal from the first fiber and separates the wavelength division multiplexed into a plurality of separate light signals. A plurality of phase conjugators correspond, respectively, to the plurality of separate light signals. Each phase conjugator receives the corresponding separate light signal from the demultiplexer and produces a corresponding phase conjugate light signal, to thereby produce a plurality of phase conjugate light signals. A plurality of second fibers correspond, respectively, to the plurality of phase conjugate light signals. Each second fiber receives the corresponding phase conjugate light signal and transmits the received phase conjugate light signal therethrough.
Additional objects and features of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.